The present invention relates to a method for producing electrostatically non-chargeable and/or electrically discharging plastic containers, and a multilayer plastic container produced with this method.
The plastic container can be formed as a jerrican or fassett, as a closed bunged drum, as lidded drum with drum lid and tension ring closure, or as an inner container of a large-volume pallet container.
State-of-the-art:
Systems in commercial use and employed in explosion protection zones or to be filled with flammable contents are, for example, pallet containers with a inner plastic container produced by a multi-layer process with a conductive and/or permanently antistatic outer layer, wherein the content is grounded in the bottom region of the container by employing suitable measures, for example by using metal screws or conductive plastics on the discharge fitting. The corresponding inner plastic containers are predominantly produced by a three-layer or six-layer co-extrusion process and have a total wall thickness of about 2-3 mm. The residual insulating wall thickness remaining after subtraction of the conductive or permanently antistatic outer layer is hereby less than 2 mm.
According to the pertinent technical literature and standards (e.g., CENELEC TR 50404, Page 10) in the technical field relating to electric devices/electronics, a wall thickness of the insulating material of less than 2 mm in conjunction with a permanently connected electrical conductor is viewed as providing adequate safety for preventing charge buildup on the opposite surface of the insulator, which would ignite contents and gases of the explosion groups Group IIA and IIB with an adequate discharge.
The self-supporting plastic containers, for example drums or jerricans, designed for use in explosion protection zones or for filling with flammable contents, are predominantly produced by a multilayer process and use a conductive and/or permanently antistatic plastic material in the outer layer. If necessary, the contents are electrically grounded by way of additionally inserted grounding elements made of conductive materials, for example by a pump pipe inserted into the container and made of electrically conductive plastic, which is electrically connected with the conductive outer layer of the container (EP 1 497 188 A).
A dischargeable plastic container is known which has a strip, similar to a viewing strip, made of conductive plastic material and introduced in the container wall (DE 102 42 955). The strip is permanently connected with the conductive outer layer and has a thickness equal to the overall thickness of the three-layer container wall. Disadvantageously, the entire container wall is completely severed in the longitudinal direction at one or several locations, because the connection of the electrically conductive plastic strip with the plastic material of the multilayer container wall always represents a critical contiguous weak spot when the filled container is subjected to greater mechanical stress.
If the total wall thickness of a container has to be increased significantly above 2 mm to provide overall stability of the container (the average wall thickness of a 8.5 kg bunged drum in the surface region is approximately 3-3.5 mm), then the need for the insulating (inner) layers to be in direct contact with the conductive outer layer with a remaining residual thickness of maximally 2.5 mm can be attained only with a correspondingly thick outer layer made of conductive material. This has significant disadvantages both due to economic and mechanical reasons, because suitable conductive plastic materials (e.g., by addition of carbon black) are relatively expensive and have significantly inferior material properties, for example with respect to their stability at low temperatures.